The present invention pertains to a whisker-reinforced ceramic and a method for making the same. More specifically, the invention pertains to a whisker-reinforced ceramic that contains aluminum oxynitride, and optionally, other materials, and a method for making the same.
Whisker-reinforced materials such as, for example, whisker-reinforced ceramics have been known for some time. In this regard, U.S. Pat. No. 4,543,345 to Wei describes a silicon carbide whisker-reinforced alumina ceramic material. U.S. Pat. No. 4,789,277 to Rhodes et al. and U.S. Pat. No. 4,961,757 to Rhodes et al. each disclose a ceramic cutting tool wherein the ceramic is a silicon carbide-whisker-reinforced alumina.
U.S. Pat. No. 6,447,896 to Augustine discloses a coated ceramic cutting tool wherein the ceramic contains silicon carbide whiskers. According to the Augustine patent, there is a listing of many ceramics suitable for use as a whisker-reinforced ceramic. These ceramics include, among many, alumina, titanium carbonitride, zirconium oxide and aluminum oxinitride.
Further, European Patent No. 0 247 630 B1 to NGK Spark Plug pertains to a ceramic material that is useful as a cutting tool. The ceramic substrate is based on a matrix composed of at least one material selected from the group consisting of Al2O3, AlN, AlON, 3Al2O3.2SiO2 (mullite) and TiC, and the substrate further comprises 5 to 50% by weight of SiC whiskers with respect to said substrate.
European Patent No. 0 861 219 B1 to Kennametal Inc. (as well as U.S. Pat. No. 5,955,390 and U.S. Pat. No. 6,204,213 B1 to Mehrotra et al. that are assigned to Kennametal Inc.) discloses titanium carbonitride-alumina-silicon carbide whisker ceramics. In these ceramics, the titanium carbonitride is the dominant matrix component.
While the above ceramic materials exhibit satisfactory properties, at least in certain situations, there still remains the need to develop new and useful ceramic materials. This is especially true for ceramic materials that are useful as cutting tools.
Heretofore, in experimental work done in the in-house facilities of Kennametal Inc. of Latrobe, Pa. 15650 USA (the assignee of the present patent application), the inventors have considered the use of aluminum oxynitride as a component of a ceramic material useful as a cutting tool. Aluminum oxynitride has mechanical properties that are similar to those of alumina, but it has a higher strength and a lower coefficient of thermal expansion than alumina. In light of these properties, the inclusion of aluminum oxynitride in a ceramic was thought to improve its thermal shock resistance. It was believed that the principal failure mechanism of alumina-silicon carbide whisker reinforced ceramic cutting tools in turning nickel-based high temperature alloys was depth-of-cut notch. There was the belief that this failure mechanism, i.e., depth-of-cut notch, was related to the thermal shock resistance and the impact strength of the cutting tool material.
Still in experimental work the inventors did in the past in the in-house facilities of Kennametal Inc., the typical way to make a ceramic material that contained aluminum oxynitride and silicon carbide whiskers was to use alumina and aluminum nitride as a part of the starting components. These starting components were hot-pressed at temperatures on the order of about 1950 degrees Centigrade. It was found that by hot pressing this powder mixture containing aluminum nitride and silicon carbide whiskers at temperatures on the order of about 1950 degrees Centigrade, the aluminum nitride and the silicon carbide whiskers formed a solid solution that caused a strong interface there between. This interface prevented whisker pull-out, i.e., pull-out of the silicon carbide whiskers from the ceramic matrix. The absence of whisker pull-out resulted in the degradation of the performance properties of the ceramic cutting tool.
In regard to whisker pull-out, U.S. Reissue Pat. No. 34,446 to Wei sets forth a discussion of whisker pull-out at Column 2, lines 30 through 53:                The use of the single crystal whiskers in the ceramic composite provide a significant improvement in the fracture toughness of the composite due to their ability to absorb cracking energy. More specifically, in a ceramic matrix where the SiC whisker-matrix interface sheer strength is relatively low as provided by radial tensile stresses across the whisker-matrix bond a process termed “whisker pull-out” occurs during cracking to absorb the cracking energy and effectively reduce the tendency to crack and also inhibit crack propagation. Whisker pull-out occurs as the matrix is subjected to crack-forming stresses. As the crack-front propagates into the composite many of the whiskers which span the crack line and extend into the ceramic matrix on opposite sides of the crack must be either fractured or pulled out of the matrix in order for the crack to grow or propagate through the ceramic. Since the single crystal SiC whiskers possess sufficient tensile strength so as to resist fracturing they must be pulled out of the matrix for the crack to propagate. As these whiskers are pulled out of the matrix they exhibit considerably bridging forces on the face of the crack and effectively reduce the stress intensity at the crack tip so as to absorb the cracking energy.        
In experimental work done by the inventors in the in-house facilities of Kennametal Inc., when titanium carbonitride was a component in the starting powder, along with aluminum nitride and alumina and silicon carbide whiskers, it was found that the titanium carbonitride and the silicon carbide considerably delayed the formation of the aluminum oxynitride. In order to overcome this consequence, the hot-pressing temperature had to be raised to such a level that the silicon carbide whiskers suffered damage.
In experimental work done by the inventors in the in-house facilities of Kennametal Inc., it was also found that when using aluminum nitride and alumina to form the aluminum oxynitride, it was difficult to control the final composition of the ceramic. More specifically, this was due to the difficulty associated with controlling the amount of aluminum oxynitride produced when using aluminum nitride and alumina in the starting powders.
Thus, it can be seen that it would be highly desirable to provide an improved whisker-reinforced ceramic material, and especially a whisker-reinforced ceramic material useful as a cutting tool, that contains aluminum oxynitride.
It can also be seen that it would be highly desirable to provide an improved whisker-reinforced ceramic material, and especially a whisker-reinforced ceramic material useful as a cutting tool, that contains aluminum oxynitride wherein the whiskers exhibit satisfactory pull-out which can be considered to be satisfactory crack-bridging pull-out from the ceramic matrix.
Further, it can be seen that it would be highly desirable to provide an improved whisker-reinforced ceramic material, and especially a whisker-reinforced ceramic material useful as a cutting tool, that contains aluminum oxynitride wherein the hot-pressing of the powder mixture occurs at a temperature low enough so as to not damage the silicon carbide whiskers and minimize the grain growth of the aluminum oxynitride.
Finally, it can be seen that it would be highly desirable to provide an improved whisker-reinforced ceramic material, and especially a whisker-reinforced ceramic material useful as a cutting tool, that contains aluminum oxynitride wherein there is control over the amount of aluminum oxynitride in the ceramic.